Integrated circuits (IC) used in wireless or other communications generally comprise a frequency synthesis means. The phase locked loop (PLL) is a common method of frequency synthesis. A PLL normally includes a phase detector and a voltage controlled oscillator (VCO). A PLL can also include a divider, loop filter, or a number of other devices depending on the intended output frequency of the PLL.
In transceivers for wireless communications, usually a mixer is used for frequency upconversion. In the mixer, the baseband, information-bearing signal is “mixed” with the local oscillator signal from the aforementioned frequency synthesizer to generate a passband signal, which is buffered by an on-chip amplifier and sent to an external power amplifier.
One reason for using the power amplifier externally is that unwanted interaction between the power amplifier and the rest of the transmission circuitry, such as the frequency determining circuitry, can cause detrimental effects, such as frequency pulling. Frequency pulling can manifest itself as errors in the output of the frequency determining circuitry due to unwanted interaction between the PLL and the PA. These errors in the frequency can lead to a receiver being unable to receive the transmitted signal.
Frequency pulling generally occurs because the PA and the PLL are operating at similar frequencies which can cause interaction between signals in the two circuits. This problem is mitigated in the current generation of designs by introducing modulation between the frequency determining circuitry and the PA. By way of example, the PLL could be operating at 2 GHz and the PA could be operating at 1 GHz. That way, the harmful emissions from the PA will be well below the operating frequency of the PLL, causing no harmful interaction between the two.
However, this modulation, or dividing or multiplying of the frequency, can cause spurious outputs from the divider. Spurious outputs are caused by non-ideal parameters inherent to substantially all integrated circuits, such as non-linearity and mismatching. These spurious outputs have frequency components that can violate the transmission mask. The transmission mask, or spectral mask, is a mathematically defined set of lines applied to the levels of radio (or optical) transmissions. The spectral mask is generally intended to reduce interference with other wireless apparatuses that are physically close to the transmitter by limiting excessive radiation at frequencies beyond the necessary bandwidth. Furthermore, transmission masks of a certain shape for a given wireless communications appliances are usually mandated by the FCC. Attenuation of these spurious emissions is usually done with a bandpass filter, also known as a transmission (TX) filter, tuned to allow the correct frequencies of the transmitted signal through an antenna. Additionally, by placing the TX filter after the power amplifier, the TX filter must be of a higher order to obtain equivalent overall suppression of the spurious outputs. Higher order filters typically cause some loss in the desired passband, which has a detrimental effect on the overall efficiency of the system. In other words, more electric power is required to transmit an equivalent filtered signal than an equivalent unfiltered signal.
The fact that the power amplifier is external and separate further exposes the fundamental problems with the current solution. Each individual chip will typically require several passive external components such as resistors and capacitors, increasing the bill of materials and the complexity of the resulting circuit. The cost rises not only due to the materials themselves but also due to the increased assembly and test time the additional units require. Also, in the prior art, the PA is usually implemented in differential form to prevent any spurious outputs at the PA output to interfere with the PLL. Differential PAs consume greater power than their single ended counterparts, leading to lower overall efficiency in the integrated circuit.
FIG. 1 shows certain aspects of the prior art and its shortcomings. The PLL 100 determines a differential output frequency 105 which is coupled as an input to a frequency divider 110. The output of the frequency divider 110 is coupled to a mixer 120. The frequency divider 110 is configured to transmit and the mixer 120 is configured to receive a divided differential frequency 115. As an unwanted consequence of frequency dividing, the divider also transmits spurious outputs 125. The mixer 120 combines this divided differential frequency 115, including spurious outputs 125 with an input signal 130. The input signal 130 can be voice, or data, or both. The mixer 120 transmits, and the differential to single ended converter (D2S) 140 is configured to receive, a differential modulated signal 135. The spurious outputs 125 are also undesirably transmitted with the differential modulated signal 135. In current practice, the rest of the TX chain is external to the integrated circuit. The D2S 140 transmits, and an external PA 150 is configured to receive, a single ended signal 145. The D2S 140 also transmits and the external PA 150 also receives the unwanted spurious outputs 125. The respective magnitudes of the single ended signal 145, which is the desired signal, and the spurious outputs 125 are represented as spikes between the D2S 140 and PA 150. The external PA 150 amplifies the single ended signal 145 but also amplifies the unwanted spurious output 125. The external PA 150 transmits, and a TX filter 160 is configured to receive, an amplified signal 155. However, the TX filter 160 also receives unwanted amplified spurious outputs 157. The respective magnitudes of the amplified signal 155, which is the desired signal, and the amplified spurious outputs 157 are represented as spikes between the PA 150 and TX filter 160. A person of ordinary skill in the art of integrated circuit design will appreciate that the difference in magnitude between the amplified signal 155 and amplified spurious output 157 will be less than the difference in magnitude between the single ended signal 145 and spurious output 125. In case of high efficiency power amplifiers, such as class AB or class C, unwanted spurious spurious signals experience much higher gain compared to the desired signal 155 due to gain compression in the power amplifier. Rapid growth of the spurious signals by such non-linear (yet highly efficient) PAs is the main reason why presence of a TX filter after the PA is unavoidable. The TX filter 160 usually is a filter of multiple orders to sufficiently attenuate the amplified spurious outputs 157 such that the transmission mask conforms to a predetermined limit or an FCC limit. The TX filter 160 transmits, and an antenna 170 is configured to receive, a filtered signal 165. Because of the multiple orders of filtering, the TX filter 160 will also undesirably attenuate the amplified signal 155. To compensate for this, the bias current of the external PA 150 will have to be increased so that its output power will increase, causing an overall decrease in efficiency and battery life.
FIG. 1A is a representation of such spurious outputs in the frequency domain. By way of example, a desired differential output signal 190 is formed in the spectrum. Non-ideal characteristics, such as non-linearity and imperfect matching that are inherent to most integrated circuits form spurious outputs 195. The spurious outputs 195 can be random or they can be a harmonic of the desired signal 190. Regardless of the way that they are formed or whether they are random, harmonic or periodic, spurious outputs are preferably suppressed for optimum performance. If not suppressed, the spurious outputs can interfere with not only the rest of the circuitry on the integrated circuit, but also other integrated circuits in the appliance, or other appliances nearby.
The difficulties that arise in integrating the PLL and PA together discussed above require a new architecture along with corresponding circuits to realize a PLL and a PA, configured to work at present day wireless communication frequencies, integrated onto one semiconductor substrate.